A multi-color sheet-fed printing machine generally includes a plurality of printing units arranged such that each printing unit imprints a partial image of a predefined color on the sheet being printed. Typically, these printing units are connected to one another by a train of driving gears to achieve synchronization between the units. It is also common to drive the driving gears associated with the individual printing units with a single longitudinal drive shaft which is connected to the train of driving gears by a plurality of input drives. Thus, the drive of a printing machine of this type usually includes a drive shaft, a plurality of input drives, a train of driving gears, and a motor for rotating the drive shaft. One common problem encountered in this type of drive is backlash within the gear train. Such backlash can create irregularities in the print quality by causing the partial images imprinted by different printing units to be printed out of registry, resulting in poor print quality.
In order to eliminate these problems, it is known to take measures to eliminate backlash by applying a pre-stress to the drive. For example, a drive for multi-color, sheet-fed rotary printing machines in a series arrangement having at least two printing units is disclosed in German Patent Document DE 2,340,263. In this document, the disclosed drive has a longitudinal shaft and two worm gears which connect the longitudinal shaft to the train of gears of the individual printing units. To eliminate backlash, the longitudinal shaft is divided up and the individual parts are connected via an axially movable clutch. A hydraulic device can be employed to adjust the axially movable clutch to thereby tension the drive in the region of the worm gears to compensate for backlash in the gear train.
Further examples of pre-stressing the drive assembly to eliminate backlash can be found in Swiss Patent 570,266 and German Patent Document DE 2,354,541. In the former, a drive for a multi-color, sheet-fed rotary printing machine with series construction of the printing units is disclosed. This drive has a longitudinal shaft which is divided up into parts which are connected to one another by a torsion bar. A specially arranged gear is utilized to pre-stress this torsion bar in order to eliminate backlash in the drive. German Patent Document DE 2,354,541, on the other hand, discloses a drive for a multi-color, sheet-fed rotary printing machine having at least two printing units connected. In this case, a longitudinal shaft with associated worm gears for driving the individual printing units is pre-stressed axially in order to eliminate the backlash in the worm gears.
All of the above-described pre-stressing arrangements are limited in that they are either designed to act permanently on the drive of an associated printing unit, or, they are designed to work in printing machines where all of the individual printing units are permanently connected to one another. There are, however, applications in which the printing units in a printing machine must be adjusted relative to each other from time to time. For example, some printing machines are designed to do both single-sided printing and double-sided printing. Single-sided printing is often called "face printing", and double-sided printing is often called "perfecting." In the double-sided printing mode, a sheet-turning device in the printing machine turns the sheet over after the first side is printed so that printing of the second side can be effected. Every time the printing mode of the machine is changed from the single-sided printing mode to the double-sided printing mode or vice versa, it is necessary to adjust a number of printing units in the printing machine relative to one another depending on the position of the sheet turning point and the sheet format being used. For this purpose, a separable connection of the drive is provided at a point in the train of gears. By releasing a clutch arranged at this turning point, the printing units ahead of the turning point can be rotated relative to the printing units behind the turning point and vice versa. This adjustment causes the backlash conditions between the separated groups of printing units to change. Because the backlash elimination arrangements discussed above are designed to compensate for fixed backlash conditions, they are not useful in a printing machine where the individual printing units are separated to adjust the printing mode from time to time. Since a continuous connection of the drive parts via a longitudinal shaft is not possible with the known arrangements for converting a printing machine from face printing to perfecting and vice versa, there is a need for providing a method for eliminating the backlash associated with such a machine.